Question

I am generating form and handling the submit event in the same file.

If user has not entered the title, I want to display the form again and include an error message (e.g. “You forgot the title.”).

That means that I have to duplicate code twice – once to diplay empty form and second to display form with body and ask user to enter title:

<?php if(strlen(strip_tags($_POST['posttitle'])) == 0):
    // Display the form with question body that user has entered so far and ask user to enter title.
?>
    <label for="title"><b>Title:</label><br/>
    <input type="text" name="posttitle" id="posttitle" />           
<?php endif;?>

<?php elseif ( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action']) && $_POST['action'] == 'post') : ?>
<!-- Everything ok - insert post to DB -->
<?php else : 

   // just display form here (again ouch!)
    <label for="title"><b>Title:</label><br/>
    <input type="text" name="posttitle" id="posttitle" />
?>

Answer 1

I would do it like this:

If REQUEST_METHOD is POST I will validate the input and collect messages in an array ($errors in my code).

Then I would just print the form and if there was an error the code will print it.

<?php

$errors = array();

function print_value_for($attr) {
    if (isset($_POST[$attr]))
        echo $_POST[$attr];
}

function print_error_for($attr) {
    global $errors;

    if (isset($errors[$attr]))
        echo $errors[$attr];
}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    // do validation here and add messages to $errors
    // like $errors['posttitle'] = "The title you entered is bad bad bad";

    if (empty($errors)) {
        // update database and redirect user
    }
}
?>

<!-- display the form and print errors if needed -->
<form>
    <?php print_error_for('posttitle'); ?>
    <input name="posttitle" type="text" value="<?php print_value_for('posttitle') ?>">

    <?php print_error_for('postauthor'); ?> 
    <input name="postauthor" type="text" value="<?php print_value_for('posttitle') ?>">

    <?php print_error_for('postbody'); ?>   
    <textarea name="postbody">
        <?php print_value_for('posttitle') ?>
    </textarea>

    <input type="submit">
</form>

PS. Consider using MVC to separate code and templates.